It has become possible only recently for telephone company customers to install and use their own station apparatus in conjunction with a telephone line extending between the telephone company's central office and the customer station. Usually, the telephone company can exercise little or no control over the type and quality of such customer station apparatus and accordingly can assume no responsibility for its maintenance or operation. When a fault appears on the telephone line, therefore, it is desirable for the telephone company to determine precisely whose apparatus is causing the fault in advance of the telephone company sending a repairman to the customer station location to avoid the expense of the trip for which the telephone company will not be compensated if the fault has originated in the customer station apparatus.
The prior art teaches line lifting apparatus, a portion of which is located at the customer station, and which is operative to lift or disconnect the telephone line from the customer station apparatus at the points of interconnection thereof.
In one such apparatus, a pair of like-poled diodes interconnect, respectively, tip and ring conductors of the telephone line with a time delay circuit whose output is connected through a relay coil to ground. The relay contacts, in a de-energized condition of the relay, connect the tip and ring conductors to the customer station apparatus. When a test voltage is applied at the central office from either the tip conductor or the ring conductor to ground, one of the diodes will conduct, thereby actuating the time delay circuit which thereafter maintains the relay coil energized for a predetermined period of time. Energization of the relay coil causes the contacts thereof to disconnect the tip and ring conductors of the telephone line from the customer station apparatus and to connect the tip and ring conductors to a test apparatus, typically comprising a termination circuit including a series R-L network. Thereafter, testing signals are applied to the tip and ring conductors at the central office to measure the impedance between the lines and to carry out other tests. If the result of the tests carried out is negative, it can be assumed that the fault exists in the customer station apparatus and therefore an unnecessary trip can be avoided.
Such prior art line lifting apparatus encounters many problems in application. For example, if a ground fault exists on the telephone line at any point, the time delay circuit generally cannot be actuated by the application of a test voltage, thereby necessitating a trip to the customer station location even though the ground fault may exist in the customer station apparatus. Further, the time delay circuits of the prior art must typically include an energy storage device such as a capacitor which is charged by the test voltage and which is discharged through the relay coil to provide the predetermined period of relay actuation. Due to physical limitations on the size of such capacitors, the predetermined period of time of relay energization often is not long enought to permit the necessary tests to be conducted upon the line. In addition, the time delay circuit often adversely affects the tests that can be carried out on the line due to the fact that the time delay circuit must necessarily have a low impedance to permit the capacitor to be charged to a value sufficient to allow the relay to be actuated for even a short time. Finally, the time delay circuit, due to its relatively low impedance, often adversely affects ringing signals impressed on the line to a point where conventional ring-trip relays in the central office often detect the presence of the time delay circuit as an off-hook condition signifying that a telephone instrument has been picked up at the customer station when in fact such is not the case. In some cases, the time delay circuit may also distort dial pulses impressed on the line.
It is therefore an object of the present invention to provide a line lifting apparatus which permits a telephone line to be lifted for whatever time is required to carry out any and all tests on the line.
It is another object of this invention to provide such a line lifting apparatus which will function to lift the telephone line even though there is a ground fault on the telephone line from either the ring conductor or the tip conductor to ground.
It is yet another object of this invention to provide such a line lifting apparatus which has a relatively high impedance with respect to ground so as to not adversely affect tests conducted on the line and so as to not adversely affect ringing signals and dial pulses impressed on the line.